The invention relates to improved heat exchangers, providing increased heat exchange and faster operation, and particularly to exchangers used in cooling/heating devices of the Joule-Thompson type.
Fast-changing heating/cooling devices which operate on the basis of the Joule-Thompson cycle, are known in the art. A method and apparatus of refrigeration of this kind is described, for instance, in U.S. Pat. No. 4,126,017. A device of this kind particularly for cryogenic and/or surgical use, is described in co-pending Israeli Patent Application No. 104506. While this invention is not limited to any particular type of heat exchanger and it should be understood that it has a general applicability, it is particularly useful for any heat exchangers used in apparatus of the Joule-Thompson type.
Such heat exchangers comprise a first duct which is at a high pressure and a second duct which is at a low pressure, the two ducts being in heat-exchanging relationship to one another. If the apparatus is a cooling device or a refrigerator, the working refrigerant fluid flows through the high pressure duct and the expanded refrigerant fluid flows through the low pressure duct. In a typical arrangement of such heat exchangers, the low pressure duct is an annular space between two tubular bodies and the high pressure duct is a pipe of small diameter, e.g. with an inner diameter comprised between 0.2 and 0.5 mm and a thickness comprised between 0.1 and 0.2 mm, helically disposed in said annular space and provided with fins. The expanded gas flows through said annular space and comes into contact with the outer surfaces of the fins, thus exchanging heat with the gas flowing within the high pressure pipe. The fins have a determining influence in providing good heat exchange.
In a type of heat exchanger of the prior art, the fins are provided by a copper wire having a rectangular cross-sectionxe2x80x94hereinafter designated as xe2x80x9cribbonxe2x80x9dxe2x80x94which is wound helically about the high pressure pipe, with its longer side extending radially from the high pressure pipe and its shorter side being in contact with said pipe. The ribbon thus constitutes a continuous helical fin, immersed in the low pressure space. Each turn of the helical fin is equivalent to and may be considered as an individual annular fin, so that the helical fin is equivalent to a plurality of fins spaced from one another. Firmly to attach the ribbon, constituting said fin, to the outer surface of the high pressure pipe, a coating of tin is electrolytically applied to the copper ribbon and to the outer surface of the high pressure pipe, to bind them together.
While the performance of existing heat exchangers, so constructed, is not unsatisfactory, their performance is not as good as desirable, particularly as to the speed of the cool-down cycles.
It is therefore a first purpose of this invention to improve heat exchangers by improving their thermal performance.
It is another purpose of the invention to improve heat exchangers by rendering them capable of producing a quicker heat exchange, in the sense of a faster cool-down.
It is a further purpose of the invention to provide improved heat exchangers by improving their mechanical characteristics, in particular the connection high pressure pipes and the heat-conductive fins applied to their exterior.
It is a still further purpose of this invention to provide all such improvements in a very simple manner.
It is a still further of the invention to provide all the aforesaid improvements with particular reference to heat exchangers used in apparatus operating by the Joule-Thompson cycle.
Other purposes and advantages will appear as the description proceeds.
The improved heat exchanger according to the invention is characterized in that it comprises a pipe, in particular a high-pressure pipe, and heat-conductive fins, that are connected to the pipe by a coating of silver.
Preferably, the fins are constituted by a copper wire, more preferably, a rectangular cross-section, viz. a copper ribbin, helically wound about the pipe.
According to an aspect of the invention, the silver coating is produced electrolytically. Therefore, the invention provides a method for making a heat exchanger which comprises the steps of providing a pipe, providing a copper wire, preferably of a rectangular cross-section, viz. a copper ribbon, winding said wire helically about said pipe, and electronically applying a coating of silver to join said wire to said pipe.
Preferably, the pipe is made of a copper-nickel alloy and has an inner diameter comprised between 0.2 and 0.5 mm and a thickness comprised between 0.1 and 0.2 mm. Also preferably, the copper wire has a rectangular cross-section, the longer side of which is comprised between 0.1 and 0.3 mm and the short side of which is comprised between 0.05 and 0.2 mm. Preferably, the rectangular copper wire is wound about the pipe in such a way that its longer side is perpendicular to the pipe, while one of its shorter sides contacts the pipe.
In particular, the heat exchangers having the structure according to the invention and produced by the method of the invention are useful as heat exchangers for apparatus operating by the Joule-Thompson cycle, e.g., cryogenic and/or surgical apparatus.